Automatic gain control circuit

ABSTRACT

An automatic gain control circuit which includes an amplifier portion for amplifying the automatic gain control signal, an impedance control portion driven by the output of the amplifier portion, a variable impedance portion provided separately from the amplifier portion and controlled by the impedance control portion, and an AC bypass portion arranged on the input side of the impedance control portion, whereby even when the control signal is at a low level, the gain control can be effected at a low distortion factor.

United States Patent [191 51 Nov. 5, 1974 Takahashi et al.

[ AUTOMATIC GAIN CONTROL CIRCUIT [75] Inventors: Kazuya Takahashi,Katsuta; Mitsuya Sato, Tokyo; Kaname Ohta, Katsuta; Tadashi Takamiya,

, Yokohama, all of Japan [73] Assignee: Hitachi, Ltd., Tokyo, Japan [22]Filed: Aug. 7, 1973 21 Appl. No; 386,415

[30] Foreign Application Priority Data UNITED STATES PATENTS 3,397,3248/1968 Fine et al. 330/29 X Primary Examiner-Herman Karl SaalbachAssistant Examiner-James B. Mullins Attorney, Agent, or FirmCraig &Antonelli [57] ABSTRACT An automatic gain control circuit which includesan amplifier portion for amplifying the automatic gain control signal,an impedance control portion driven by the output of the amplifierportion, a variable impedance portion provided separately from theamplifier portion and controlled by the impedance control portion, andan AC bypass portion arranged on the input side of the impedance controlportion, whereby even when the control signal is at a low level, thegain control can be effected at a low distortion factor.

3,267,388 8/1966 Finkey et al. 330/29 X 17 Claims, 5 Drawing FiguresPRIOR ART ,A PA T |N PRE AIVPL 2%? g Vcc Y SP T NJTOTAATICTGAIN CONTROLCKT pmmgnauv 51974 3.846711 FIG.

{A {PA T |N-PREAIvPL mi? PRIOR ART FIG. 2

PRIOR ART Tl v TI 1 AUTOMATIC GAIN CONTROL CIRCUIT BACKGROUND OF THEINVENTION 1. Field of the Invention The present invention relates to anautomatic gain control circuit, and more particularly to an automaticgain control circuit suitable for fabrication as an integratedsemiconductor circuit.

2. Description of the Prior Art As shown in FIG. 1, a conventional voiceamplifier device is constructed such that a voice input signal suppliedto an input terminal IN is amplified by a preamplifier portion A, andthe amplified input signal is fed through a power amplifier portion PAto a speaker SP. In this case, a matching transformer T is arrangedbetween the power amplifier portion PA and the speaker SP, so as toeffect impedance matching between the.

amplifier and the speaker. In order to provide automatic gain control,an automatic gain control circuit AGC is arranged between the outputside and the input side 'of the power amplifier portion PA. In thefigure, R, and R designate resistances, and T, T designate terminals ofthe automatic gain control circuit AGC.

The automatic gain control circuit AGC generally I employed in thesystem of FIG. I is composed, as illustrated in FIG. 2, of an n-p-ntransistor Q, and three diodes D, D which are connected in series in theforward direction to the emitter of the transistor 0,. One end of thediode D is grounded through the terminal T,, while the collector of thetransistor Q, is connected to a power source V through the terminal T,.Supplied through the terminal T, to the base of the transistor Q, is anautomatic gain control signal Sc, which is produced in such a way thatan AC signal derived from the output side of the power amplifier portionPA in FIG. I is rectified and smoothed by means of a detecting diode D,and -a capacitor C,. The juncture P, between the diodes D, and D isconnected through the terminal T, as well as a capacitor C to the inputside of the power amplifier portion PA.

Depending on the magnitude of the control signal Sc, supplied to thebase of the transistor 0,, the impedance of the diodes D, D varies, sothat the impedance component included in the input of the poweramplifier portion PA changes. Thus, the gain control of the device iscarried out.

With such a construction, gain control is possible with a small controlcurrent. Furthermore, the input signal to be controlled is subject topush-pull operation, so that even-order higher harmonics are removed,making gain control with a low distortion factor possible. On the otherhand, however, the voltage to be supplied to the base of the transistorQ, must be made higher than the sum of the rise voltages of thetransistor Q, and the diodes D, D in order to reduce the distortionattributable to the non-linearity of the rise in the voltage-currentcharacteristics of the transistor Q, and

the diodes D, D In particular, since the transistor 0,

also has the detecting diode D connected on its input side, theeffective voltage required for automatic gain control is approximately 3V or higher.

For this reason. according to the prior art, a special winding fortaking out the automatic gain control signal is provided on thesecondary side of the matching transformer T disposed on the output sideof the power amplifier portion PA.

- When it is intended to utilize the automatic gain control circuit AGCof the foregoing construction with a voice amplifier device of the OTLsystem, the voltage for automatic gain control cannot be set to a highlevel, as a result of which the circuit is subjected to an increaseddistortion.

SUMMARY OF THE INVENTION It is therefore a principal object of thepresent invention to provide an automatic gain control circuit whichenables gain control with a low distortion factor even with a controlsignal of low level.

Another object of the present invention is to provide an automatic gaincontrol circuit which is suitable to be provided in the form of anintegrated semiconductor circuit.

In order to accomplish such objects, the present invention consists of acircuit in which an automatic gain control signal is amplified afterdetection. The amplifier portion therefor and a variable impedanceportion are separated, the latter being controlled by an impedancecontrol portion which is driven by the output of the amplifier portion.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic circuit diagramalready referred to, showing an example of a prior-art voice amplifierdevice;

FIG. 2 is a schematic circuit diagram already referred to, showing anexample of the automatic gain control circuit in FIG. I; and

FIGS. 3 to 5 are schematic circuit diagrams each showing a respectiveembodiment of the automatic gain control circuit according to thepresent invention.

PREFERRED EMBODIMENTS OF THE INVENTION FIG. 3 shows an embodiment of theautomatic gain control circuit according to the present invention. Inthe figure, Q Q, designate n-p-n transistors, 0,, and Q designate p-n-ptransistors, and R designates a resistance. The transistor Qconstituting an amplifier portion for an automatic. gain control signalhas its emitter connected through the resistance R, to a terminal T,leading to ground, and has its collector connected through the collectorand emitter of the transistor O to a power source terminal T,. The baseof the transistor Q, is connected to its collector electrode and to thebase of the transistor Q The emitter of the transistor 0,, is connectedto the power source terminal T,, while the collector is connected to thebase of the transistor Q constituting an impedance control portion. Thecollector of the transistor O is connected to the power source terminalT,, while the emitter is connected to a terminal T, (on the input sideof the power amplifier portion in FIG. I) and also to ground at theterminal T, through the collector-emitter junction of the transistor 0,.The transistor 0, has its base connected to its collector, thusconstituting a variable impedance diode. Further, the transistors O, andQ, are

With'such a construction, the detected automatic gain control signal Sc,applied to the terminal T (the input terminal of the automatic gaincontrol circuit) is sufficiently DC-amplified by the transistor Q Theamplified signal is applied through the transistors Q and Q, to the baseof the transistor Q Accordingly, a voltage sufficiently higher than therise voltage V of the transistor 0,, is applied to the transistor Qwhich is connected on the emitter side of the transistor 06 and whichconstitutes a variable impedance diode. For this reason, the transistorQ operates in an operating region of little non-linear distortion. Inconsequence,

even if the automatic gain control signal Sc, applied to the terminal Tis at a low level, a gain control of low distortion factor can beeffected.

Across the transistors Q and Q connected in series and respectivelyconstituting the AC bypass diodes, there isapplied a voltage which ishigher than double the rise voltage V The dynamic resistances of thesetransistors Q and Q, are accordingly very small, so that the base of thetransistor 0., for controlling the gain is grounded AC-wise.Consequently, the transistors and Q 'can effect a substantially perfectpush-pull operation for the input signal to-be-controlled. For thisreason, even if the automatic gain control signal is at a low inputlevel, it is possible to achieve a low distortion factor.

With the construction of the present invention as described above, eventhough the detector portion including diode D, for detecting the signalderived from transformer T for automatic gain control and the impedancecontrol portion formed by the transistor Q6 are separated by theamplifier portion formed by tranployed. Accordingly, if the automaticgain control circuit is constructed as an integrated semiconductorcircuit (lC), terminals otherwise fitted outside the IC device forconnecting such a capacitor are unnecessary, and therefore, the numberof terminals required for external connection to the 1C device need notbe especially increased. 7

FIG. 4 shows another embodiment of the automatic gain control circuitaccording to the present invention. In this case, diodes D and D areemployed in place of the transistor 0 in FIG. 3, and diodes D D are usedin place of the transistors Q and 0 With such a construction, the riseof the voltage-current characteristic of the diodes D and D constitutingvariable impedance diodes becomes smaller than in the case of thecircuit of FIG. 3. This results in less distortion.

Herein, a diode D is connected to the emitter side of the transistor Qso that, when the transistor Q and the diodes D and D perform the pushpull operation for the input signal to-be-controlled, the impedances forthe positive and negative cycles of the input signal to-be-controlledmay be matched to reduce the distortron.

The four diodes D D constituting AC bypass diodes are employed so thatthe base of the transistor Q can be grounded AC-wise by making the sumofthe rise voltages of the transistor 0,, and the diodes D D and D andthe sum of the rise voltages of the diodes D 4 D substantially equal.The AC resistance of these diodes D, D is selected to be sufficientlysmaller than the AC resistance of the transistor 0.; and the diode D orthat of the diodes D and D FIG. 5 shows still another embodiment oftheautomatic gain control circuit according to the present invention. inthis case, a load resistance R, is substituted for the transistors Q8and O in FIG. 4. Even withsuch a construction, substantially the sameoperation as in the foregoing cases can be effected.

Although, in the embodiments described above, the bipolar transistor isemployed as the impedance control portion, it may, of course, bereplaced with a MOS field-effect transistor.

Although, in the embodiments, the transistors or usual diodes havingrise voltages are used as the AC bypass portion, it may be constitutedof any semiconductor device having a rise voltage. For example, a Zenerdiode may also be used. in this case, the semiconductor device shouldpreferably have the lowest possible AC resistance in a region ofvoltages above the rise voltage.

Although, in the embodiments, the transistor or usual diodes having risevoltages are employed as the variable impedance portion as in the ACbypass portion referred to above, they may be replaced with any othersemiconductor device having a rise voltage.

As described above, in accordance with the automatic gain controlcircuit of the present invention, a gain control of low distortionfactor can be effected even for an automatic gain control signal of lowlevel. Besides, when the present invention is applied to a voiceamplifier device of, for example, the OTL system which employs anintegrated semiconductorcircuit, a capacitor between the automatic gaincontrol circuit and the power amplifier portion can be dispensed with.It is therefore possible to decrease the number of terminals to beexternally provided for the integrated circuit, and to obtainminiaturization of the device.

What is claimed is:

1. An automatic gain control circuit comprising amplifier means foramplifying an automatic gain control signal, variable impedance meanshaving a non-linear voltage characteristic for providing a varyingimpedance in response to the amplified automatic gain control signal, animpedance control circuit connecting the output of said amplifier meansto said variable impedance means, and bypass means including a pluralityof forward biased PN junction devices series connected to the input ofsaid impedance control circuit and having a non-linear voltagecharacteristic for bypassing AC signal components to ground.

2. An automatic gain control circuit as defined in claim 1, wherein saidamplifier means includes a voltage source, a first transistor connectedto said voltage source and having its input electrode connected to aninput terminal receiving said automatic gain control signal.

3. An automatic gain control circuit as defined in claim 2, wherein saidamplifier means further includes a second transistor connected betweensaid first transistor and said impedance control circuit.

4. An automatic gain control circuit as defined in claim 2, wherein saidamplifier means further includes a first resistor connected between saidfirst transistor and said voltage source and a second resistor connectedbetween said first transistor and ground.

5. An automatic gain control circuit as defined in claim 4, wherein saidimpedance control circuit includes a second transistor connected betweensaid voltage source and an output terminal and having an input electrodeconnected to the output of said amplifier means.

6. An automatic gain control circuit as defined in claim 5, wherein saidimpedance control circuit further includes a diode connected betweensaid second transistor and said output terminal.

7. An automatic gain control circuit as defined in claim 6, wherein saidvariable impedance means includes a first pair of diodes connected inseries between said output terminal and ground.

8. An automatic gain control circuit as defined in claim 7, wherein saidPN junction devices of said bypass means include second and third pairsof series connected diodes connected in series between the inputelectrode of said third transistor and ground.

9. An automatic gain control circuit as defined in claim 1, wherein saidPN junction devices of said bypass means include a plurality of diodesconnected in series between the input of said impedance control circuitand ground.

10. An automatic gain control circuit as defined in claim 1, wherein thenumber of PN junction devices provided in said bypass means is equal tothe number of forward biased PN junctions comprised in said variableimpedance means and said impedance control circult.

11. An automatic gain control circuit comprising amplifier means foramplifying an automatic gain control signal, variable impedance meanshaving a non-linear voltage characteristic for providing a varyingimpedance in response to the'amplified automatic gain control signal, animpedance control circuit connecting the output of said amplifier meansto said variable impedance means, and bypass means connected to theinput of said impedance control circuit and having a nonlinear voltagecharacteristic for bypassing AC signal components to ground, whereinsaid impedance control circuit includes a first transistor connectedbetween a voltage source and an output terminal and having an inputelectrode connected to the output of said amplifier means, said variableimpedance means being connected to said output terminal in series withsaid impedance control circuit across said voltage source.

12. An automatic gain control circuit as defined in claim 11, whereinsaid impedance control circuit further includes a diode connectedbetween said first transistor and said output terminal.

13. An automatic gain control circuit as defined in claim 11, whereinsaid variable impedance means includes a second transistor connectedbetween said output terminal and ground, the base of said secondtransistor being connected to the collector thereof.

14. Anautomatic gain control circuit as defined in claim 13, whereinsaidbypass means includes third and fourth transistors connected in seriesand each having its base connected to its collector.

15. An automatic gain control circuit as defined in claim 11, whereinsaid variable impedance means includes a first pair of diodes connectedin series between said output terminal and ground.

16. An automatic gain control circuit as defined in claim 15, whereinsaid bypass means includes second and third pairs of series connecteddiodes connected in series between the input electrode of said firsttransistor and ground.

17. An automatic gain control circuit as defined in claim 11, whereinsaid variable impedance means includes a first pair of diodes connectedin series between said output terminal and ground.

* l =l= l l

1. An automatic gain control circuit comprising amplifier means for amplifying an automatic gain control signal, variable impedance means having a non-linear voltage characteristic for providing a varying impedance in response to the amplified automatic gain control signal, an impedance control circuit connecting the output of said amplifier means to said variable impedance means, and bypass means including a plurality of forward biased PN junction devices series connected to the input of said impedance control circuit and having a non-linear voltage characteristic for bypassing AC signal components to ground.
 2. An automatic gain control circuit as defined in claim 1, wherein said amplifier means includes a voltage source, a first transistor connected to said voltage source and having its input electrode connected to an input terminal receiving said automatic gain control signal.
 3. An automatic gain control circuit as defined in claim 2, wherein said amplifier means further includes a second transistor connected between said first transistor and said impedance control circuit.
 4. An automatic gain control circuit as defined in claim 2, wherein said amplifier means further includes a first resistor connected between said first transistor and said voltage source and a second resistor connected between said first transistor and ground.
 5. An automatic gain control circuit as defined in claim 4, wherein said impedance control circuit includes a second transistor connected between said voltage source and an output terminal and having an input electrode connected to the output of said amplifier means.
 6. An automatic gain control circuit as defined in claim 5, wherein said impedance control circuit further includes a diode connected between said second transistor and said output terminal.
 7. An automatic gain control circuit as defined in claim 6, wherein said variable impedance means includes a first pair of diodes connected in series between said output terminal and ground.
 8. An automatic gain control circuit as defined in claim 7, wherein said PN junction devices of said bypass means include second and third pairs of series connected diodes connected in series between the input electrode of said third transistor and ground.
 9. An automatic gain control circuit as defined in claim 1, wherein said PN junction devices of said bypass means include a plurality of diodes connected in series between the input of said impedance control circuit and ground.
 10. An automatic gain control circuit as defined in claim 1, wherein the number of PN junction devices provided in said bypass means is equal to the number of forward biased PN junctions comprised in said variable impedance means and said impedance control circuit.
 11. An automatic gain control circuit comprising amplifier means for amplifying an automatic gain control signal, variable impedance means having a non-linear voltage characteristic for providing a varying impedance in response to the amplified automatic gain control signal, an impedance control circuit connecting the output of said amplifier means to said variable impedance means, and bypass means connected to the input of said impedance control circuit and having a non-linear voltage characteristic for bypassing AC signal components to ground, wherein said impedance control circuit includes a first transistor connected between a voltage source and an output terminal and having an input electrode connected to the output of said amplifier means, said variable impedance means being connected to said output terminal in series with said impedance control circuit across said voltage source.
 12. An automatic gain control circuit as defined in claim 11, wherein said impedance control circuit further includes a diode connected between said first transistor and said output terminal.
 13. An automatic gain control circuit as defined in claim 11, wherein said variable impedance means includes a second transistor connected between said output terminal and ground, the base of said second transistor being connected to the collector thereof.
 14. An automatic gain control circuit as defined in claim 13, wherein said bypass means includes third and fourth transistors connected in series and each having its base connected to its collector.
 15. An automatic gain control circuit as defined in claim 11, wherein said variable impedance means includes a first pair of diodes connected in series between said output terminal and ground.
 16. An automatic gain control circuit as defined in claim 15, wherein said bypass means includes second and third pairs of series connected diodes connected in series between the input electrode of said first transistor and ground.
 17. An automatic gain control circuit as defined in claim 11, wherein said variable impedance means includes a first pair of diodes connected in series between said output terminal and ground. 